The present invention relates generally to the repair of surface flaws and imperfections in graphite articles and more particularly to a coating formulation for filling surface irregulatories on a graphite article to effect the refurbishing of the surface thereof.
Graphite is frequently used as the construction material in the fabrication of molds, crucibles and matched die sets which are used in various metallurgical processes include the casting and pressing of uranium and uranium alloys. Often during the utilization of these graphite structures or articles the surfaces thereof become worn and deteriorated by abrasion or in some manner damaged such as by chipping, denting and the like which render the graphite articles unsuitable for their intended use. Normally these graphite articles are fabricated in precise dimensions such as in the case of molds and matched die sets and are relatively expensive to replace. Thus, efforts to repair the surface damage to these graphite articles were undertaken by applicants so as to provide an economical technique for reconditioning or refurbishing the surfaces of worn or damaged graphite articles to virtually the original surface finish and dimensions. These investigators hoped to discover a coating formulation of high purity carbon so as to be compatible with the graphite and which is capable of filling the indentation or surface imperfection to its original condition. Also, the coating material or materials must be capable of tightly adhering to the surface of the graphite article and provide the required high carbon yield upon pyrolysis. The use of high purity carbon, i.e., graphite for repairing the surface of a graphite article is required so as to restore the surface to its original condition and als prevent the introduction of possibly contaminants or other materials into the surface of the graphite article which may be reactive with the material being contacted by the graphite article during metallurgical processes. Another requirement of the filler material for repairing the surface imperfection is to provide a formulation or the like of high carbon yielding material which requires a minimal number of applications or coating layers of the formulation to the graphite surface for filling the surface imperfections of the graphite.
Initial evaluations on three coating formulations of carbon black in concentrations of 15, 25, and 50 wt % dispersed in furfuryl alcohol catalyzed with 4 wt % meleic anhydride. The formulation containing the 50 wt % carbon black was excessively thick and was diluted with acetone to enhance the application characteristics of the formulation onto a graphite surface. These formulations were each applied by brush, spatulating or troweling to the surface of a graphite article having a surface imperfection with each coating being applied in a thickness sufficient for filling the surface imperfection. Each of these coatings was found to undergo excessive cracking and spalling due to shrinkage and outgassing during carbonization. In addition to the aforementioned formulations, a commerically available alkyd resin paint having a colodial graphite pigment and obtained from Atchison Colloid Co. and sold under the trademark Dag-35 was also evaluated. In a test of this alkyd resin paint, a surface of two graphite articles was coated, cured and then lapped to original specifications. The surface finish of the graphite articles seemed to be satisfactory but when the coating was heated to pyrolysis temperature of approximately 800.degree. C. to 1000.degree. C., the shrinkage of the coating was found to be excessive and formed a surface depression in the flaw area and also caused some deterioration in the surface finish provided by the paint.